1. Field of the Invention
This invention relates to playback systems of a variable electrostatic capacitance form and more particularly to, high density information records, such as video or digital audio discs, on which signal information is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems, signal information is encoded as geometric variations or pits in a spiral plane or groove or in concentric planes or grooves on an information recording medium or substrate. The medium on which the signal information has been recorded is reproduced as follows. When a pickup stylus such as of diamond having an electrode is scanned relative to a recorded track of the medium, capative variations are established between the pickup stylus and the medium due to the geometric variations in the plane or groove.
Several types of information records have been proposed for use in the above-described type of electrostatic capacitance playback system. In one such record, an electrode is provided on the surface of a record substrate on which geometric variations are press molded according to signal information in order to permit an electrostatic capacitance to be established between the record electrode and the stylus electrode. For instance, the record substrate is covered with a thin metallic film of several hundreds angstrom in thickness and a several hundred angstrom thick dielectric layer overlying the metallic film. The dielectric layer serves to prevent short circuiting of the electrodes and increase the dielectric constant between the electrodes. The record of the above-described type needs a number of fabricating steps including a press molding of the recording medium substrate, steps of depositing the thin metallic film and the dielectric layer, and the like. Thus, the fabrication is complicate and troublesome with a need of a relatively large-scale manufacturing apparatus. Thus, production cost becomes very high.
Another type of known information record of the electrostatic capacitance form is one which is obtained by press molding a conductive plastic composition comprising a resin such as polyvinyl chloride in admixture with a conductive material such as carbon black so that signal information is recorded as geometric variations. This record is fabricated without needing the step of depositing a thin metallic film because the record itself is conductive and an electrostatic capacitance is formed between the electrode of a pickup stylus and the record. Moreover, fine carbon black powder is substantially covered with the resin, so that any dielectric layer is not necessary. Thus, this type of record is very simple in manufacture and can be manufactured at low cost.
However, when the record made of the conductive resin composition is mounted in a playback system and reproduced over a long term, its surface is worn by means of the pickup stylus contacting therewith. This results in deformation of the geometric variations or pits formed on the record, causing capative variations to change. Thus, reliable playback is not possible. Especially, when a video disc is subjected to still reproduction or freeze frame, a given portion of the disc is repeatedly contacted with a pickup stylus at a pressure of 400 to 500 Kg/cm.sup.2 at a rate of 15 times/second, causing the disc surface to be considerably worn. It will be noted that the above type of video disc is relatively poor in wear resistance not only in the still reproduction, but also in ordinary reproduction cycles and the critical number of reproduction is lower than 100.
In addition, press moldability of the conductive resin composition is not so good.
We have made extensive studies on conductive resin compositions suitable for high density information record and particularly on lubricants being added to resin compositions. It was found that some lubricants do not show satisfactory effects of lubrication and cause breeding, thus leading to a lowering in quality of the playback image. Alternatively, some lubricants produce several disadvantages in that resin compositions comprising such lubricants are not satisfactorily kneaded in continuous kneaders or extruders nor suitably molded in press molding machines.